1. Field of the Invention
The present invention relates to an air intake system for a Diesel engine and, more particularly, to an air intake system for a Diesel engine equipped with an exhaust gas recirculation system.
2. Description of the Prior Art
There have been devised and demonstrated Diesel engines equipped with exhaust gas recirculation systems (which will be referred to as "EGR" system for brevity in this specification) in order to reduce the emission of nitrogen oxides (NOx) in the exhaust gases. In a typical prior art EGR system for the diesel engine, a part of the exhaust gases is recirculated back into an intake passage or induction passage in the vicinity of a throttle valve in an air intake system. More specifically, the exhaust gases to be recirculated are introduced into the intake or induction passage through openings formed in the wall of the air intake passage. The volume of the exhaust gases to be recirculated; that is, the recirculation rate, is controlled by an EGR valve means which is rotated in unison with the throttle valve so as to vary the opening degree of the exhaust gas recirculation openings. In general, the EGR valve means comprises a pair of valve plates which are made fast to the shaft of the throttle valve for rotation in unison therewith. The valve plates and the exhaust gas recirculation openings are so designed and arranged that the exhaust gases may be recirculated into the engine over all the operation range thereof. This means that the exhaust gases are recirculated into the engine also in the throttle part-open engine operation range. This adversely affects the cold-starting of an engine and the drivability of the engine during warm up operation. Furthermore, with the cold engine, the emission of the Diesel smoke is increased.
The EGR valve means of the type described has a further problem that, because the EGR openings are located adjacent to the side edges of the throttle valve, a considerably large amount of carbon contained in the exhaust gases is deposited on the surfaces of the throttle valve and the inner sufaces of the air intake passage so that the cross sectional area of the air intake passage is reduced. As a result, particularly during engine idling operation with a maximum throttle opening, the flow of the intake or fresh air is considerably reduced as compared with the flow of the exhaust gases recirculated; that is, the ratio between the amount of the fresh air and the amount of the recirculated exhaust gases varies with the resultant variation in air-fuel ratio and the increase in the emission of the unburned gas (HC). This adverse affect caused by the deposit of carbon may be eliminated by removing carbon deposit from the throttle valve and other parts, but it is almost inpractical to request engine operators to clean the air intake system frequently because carbon is accumulated continuously and gradually as the engine is operated and, consequently, cleaning services must be made at frequent intervals.